Energy conversion systems with power control

ABSTRACT

In one embodiment, a power conversion system includes a controller to provide power control to a converter, and a distortion mitigation circuit. In another embodiment, a system includes a converter to transfer power between a power source and a load having fluctuating power demand, and a controller to provide power control, where the controller may selectively disable the power control. In another embodiment, a power conversion system includes a controller to generate a drive signal to provide power control to a power path in response to a sense signal from the power path, where the sense signal is taken from other than the input of the power path, or the drive signal is applied to the power path at other than a first power stage.

CROSS-REFERENCE TO RELATED U.S. PATENT APPLICATION

This application is a continuation application of U.S. application Ser.No. 12/368,990, entitled “Energy Conversion Systems with Power Control,”by Naiknaware et al., which was filed on Feb. 10, 2009, and which claimspriority under 35 U.S.C. §119(e) to U.S. Provisional Patent ApplicationSer. No. 61/149,305, entitled “Power Conversion with Constant PowerControl,” by Batten et al., which was filed on Feb. 2, 2009, theentirety of which is hereby incorporated by reference.

This application is a continuation-in-part of U.S. application Ser. No.12/340,715, entitled “Distributed Energy Conversion Systems,” byNaiknaware et al., which was filed on Dec. 20, 2008, and which claimspriority under 35 U.S.C. §119(e) to U.S. Provisional Patent ApplicationSer. No. 61/008,670, entitled “Distributed Energy Conversion Systems,”by Naiknaware et al., which was filed on Dec. 21, 2007, the entirety ofwhich is hereby incorporated by reference.

BACKGROUND

Power converters are used to convert electric power from one form toanother, for example, to convert direct current (DC) power toalternating current (AC) power. One important application for powerconverters is in transferring power from energy sources such as solarpanels, batteries, fuel cells, etc., to electric power distributionsystems such as local and regional power grids. Most power grids operateon AC current at a line (or mains) frequency of 50 or 60 cycles persecond (Hertz or Hz). Power in an AC grid flows in a pulsating mannerwith power peaks occurring at twice the line frequency, i.e., 100 Hz or120 Hz. In contrast, many energy sources supply DC power in a steadymanner Therefore, a power conversion system for transferring power froma DC source to an AC grid typically includes some form of energy storageto balance the steady input power with the pulsating output power.

This can be better understood with reference to FIG. 1 which illustratesthe mismatch between a DC power source and a 60 Hz AC load. The maximumamount of power available from the DC source is shown as a constantvalue. In contrast, the amount of power that must be transferred to theAC load fluctuates from zero to a maximum value and back down to minimumonce every 8.33 milliseconds (ms). During time T1, the power availablefrom the DC source exceeds the instantaneous power required by the ACload. During time T2, however, the maximum power available from the DCsource is less than that required by the load. Therefore, to effectivelytransfer power from the source to the load, the power conversion systemmust store the excess energy from the power source during time T1 (shownas the shaded area S), and discharge the stored energy to the loadduring time T2 (shown as the shaded area D).

Energy storage devices for power converters tend to be expensive, bulky,unreliable, and inefficient. These factors have been barriers tolarge-scale adoption of alternative energy sources such as solar andfuel cells which generate electricity in the form of DC power. They havealso been barriers to large scale adoption of back-up power systems forcomputers, residences, schools, businesses, etc.

The cost and reliability factors have been especially critical for solarenergy systems. Solar panel makers have improved the reliability oftheir products to the point that 20-year warranties are common.Manufacturers of power converters, however, have not reached a pointwhere they can offer warranties that are comparable to those for solarpanels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the mismatch between a DC power source and a 60 Hz ACload in a power converter.

FIG. 2 illustrates a conventional system for converting DC power from aphotovoltaic (PV) panel to AC power.

FIG. 3 illustrates power loss versus ripple voltage in a PV panel.

FIG. 4 illustrates cost versus capacitance for a capacitor.

FIG. 5 illustrates the operation of a PV power conversion system.

FIG. 6 illustrates the operation of a power conversion system havingconstant power control according to some of the inventive principles ofthis patent disclosure.

FIG. 7 illustrates an embodiment of a power conversion system havingconstant power control according to some of the inventive principles ofthis patent disclosure.

FIG. 8 illustrates another embodiment of a power conversion systemaccording to some of the inventive principles of this patent disclosure.

FIG. 9 illustrates another embodiment of a power conversion systemhaving constant power control according to some inventive principles ofthis patent disclosure.

FIG. 10 illustrates an embodiment of a controller for implementingconstant power control according to some inventive principles of thispatent disclosure.

FIG. 11 illustrates an embodiment of a power converter system accordingto some of the inventive principles of this patent disclosure.

FIG. 12 is a schematic diagram of an embodiment of a main power pathsuitable for implementing the inverter system of FIG. 11 according tosome of the inventive principles of this patent disclosure.

FIGS. 13-16 illustrate embodiments of PV panels according to some of theinventive principles of this patent disclosure.

FIG. 17 illustrates the instantaneous demand for voltage from anH-bridge type DC/AC inverter in comparison to the voltage available froma DC link capacitor that is maintained at a fixed voltage.

FIG. 18 illustrates the instantaneous demand for voltage from anH-bridge type DC/AC inverter in comparison to the voltage available froma DC link capacitor that has a large AC voltage swing due to a constantpower control feature according to some of the inventive principles ofthis patent disclosure.

FIG. 19 illustrates an embodiment of a power conversion system havingharmonic distortion mitigation according to some of the inventiveprinciples of this patent disclosure.

FIG. 20 illustrates an embodiment of a distortion mitigation systemaccording to some of the inventive principles of this patent disclosure.

FIG. 21 illustrates another embodiment of a distortion mitigation systemshowing some example implementation details according to some of theinventive principles of this patent disclosure.

FIG. 22 illustrates another embodiment of a controller having harmonicdistortion mitigation according to some of the inventive principles ofthis patent disclosure.

FIG. 23 illustrates an embodiment having grid current control accordingto some of the inventive principles of this patent disclosure.

FIG. 24 illustrates an embodiment of a controller according to some ofthe inventive principles of this patent disclosure.

FIG. 25 illustrates an embodiment of a controller having predistortionaccording to some of the inventive principles of this patent disclosure.

FIGS. 26-29 illustrate embodiments of predistortion elements accordingto some of the inventive principles of this patent disclosure.

FIG. 30 illustrates an embodiment of impedance transformation accordingto some of the inventive principles of this patent disclosure.

FIG. 31 illustrates the operation of a power conversion system withoutimpedance transformation.

FIG. 32 illustrates the voltage-current curve and power curve of atypical PV panel.

FIG. 33 illustrates V-I and power curves for a power source having morethan one local maximum power point.

FIG. 34 illustrates an embodiment of a power conversion system havingconstant power control and an input sweeping feature according to someof the inventive principles of this patent disclosure.

FIG. 35 illustrates the embodiment of FIG. 20 with the constant powercontrol disabled according to some of the inventive principles of thispatent disclosure.

FIG. 36 illustrates how the embodiment of FIGS. 20 and 21 may operateunder some conditions.

FIG. 37 illustrates an embodiment of a system with multiple powersources according to some of the inventive principles of this patentdisclosure.

FIG. 38 illustrates an embodiment of a power conversion system in whichmultiple DC/DC converters include constant power control functionalityaccording to some of the inventive principles of this patent disclosure.

FIGS. 39-42 illustrate embodiments of power conversion system havingmultiple converters with power control and a central inverter accordingto some of the inventive principles of this patent disclosure.

FIGS. 43-51 illustrate embodiments having distortion mitigationaccording to some of the inventive principles of this patent disclosure.

FIG. 52 illustrates an embodiment of a power conversion system havingEMI mitigation according to the inventive principles of this patentdisclosure.

FIG. 53 illustrates another embodiment of a power conversion systemaccording to the inventive principles of this patent disclosure.

DETAILED DESCRIPTION

FIG. 2 illustrates a conventional system for converting DC power from aphotovoltaic (PV) panel to AC power. The PV panel 10 generates a DCoutput current I_(PV) at a typical voltage V_(PV) of about 20 volts, butpanels having other output voltages may be used. A DC/DC converter 12boosts V_(PV) to a link voltage V_(DC) of a few hundred volts. A DC/ACinverter 14 converts the DC link voltage to an AC output voltageV_(GRID). In this example, the output is assumed to be 120 VAC at 60 Hzto facilitate connection to a local power grid, but other voltages andfrequencies may be used.

The system of FIG. 2 also includes a DC link capacitor C_(DC) and adecoupling capacitor C₁. Either or both of these capacitors may performan energy storage function to balance the nominally steady power flowfrom the PV panel with the fluctuating power requirements of the grid.Power pulses within the system originate at the DC/AC inverter 14, whichmust necessarily transfer power to the grid in 120 Hz pulses. In theabsence of a substantial energy storage device, these current pulseswould be transferred all the way back to the PV panel where they wouldshow up as fluctuations (or “ripple”) in the panel voltage V_(PV) and/orcurrent I_(PV). Therefore, the DC link capacitor C_(DC), or less often,the decoupling capacitor C₁, is used to store enough energy on acycle-by-cycle basis to reduce the ripple at the PV panel to anacceptable level.

In conventional systems, however, energy storage capacitors tend to beproblematic components for several reasons. First, a capacitor that islarge enough to provide adequate energy storage must generally be of theelectrolytic type, since other large capacitors are usuallyprohibitively expensive. This may be better understood in the context ofan example system that is designed to convert 210 watts of input powerfrom a PV panel to 120 VAC at 60 Hz. The energy storage AE required tobalance the power on a cycle-by-cycle basis is given by:

$\begin{matrix}{{\Delta \; E} = \frac{P}{2\omega}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

where P is the power in watts (W), ω is the angular frequency of the ACsine wave which has units of sec⁻¹, and the energy storage AE has unitsof Joules (J). At 60 Hz, ω=1207π, and thus:

$\begin{matrix}{{\Delta \; E} = {\frac{210}{2\left( {120\pi} \right)} \approx {0.3\mspace{14mu} J}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

The amount of energy stored in a capacitor is given by:

ΔE=½C[ν _(max) ²−ν_(min) ²]  (Eq. 3)

where C is the capacitance in Farads.

Assuming the energy storage function is performed in the DC linkcapacitor C_(DC), and the DC link voltage is allowed to have a 5 voltpeak-to-peak swing on top of a 495 volt DC level, solving for thecapacitance provides the following result:

$\begin{matrix}{C = {\frac{2(0.3)}{(500)^{2} - (495)^{2}} \approx {120\mspace{14mu} {\mu F}}}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

A 120 microfarad capacitor at a high enough voltage rating wouldtypically have to be an electrolytic capacitor, since a ceramiccapacitor of this size would usually be prohibitively expensive.

Using the decoupling capacitor C₁ for energy storage is typically evenworse. Since the voltage multiplication from input voltage V_(PV) to thelink voltage V_(DC) is about 25 to 1, a 5 volt peak-to-peak ripple onthe DC link would equate to a 0.2 volt ripple on the decouplingcapacitor. Solving again for the capacitance yields:

$\begin{matrix}{C = {\frac{2(0.3)}{(20)^{2} - (19.8)^{2}} \approx {75\mspace{14mu} {mF}}}} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

A 75 mF 75,000 microfarad) capacitor would almost certainly need to beof the electrolytic type.

Electrolytic capacitors, however, have limited life spans and tend tohave a high failure rate. As a further complication, the capacitance ofan electrolytic capacitor steadily decreases over its lifetime as theelectrolyte dissipates and/or deteriorates, thereby reducing itseffectiveness and changing the dynamics of the entire system. Further,electrolytic capacitors tend to be bulky, heavy and fragile, and have alarge equivalent series resistance (ESR).

As is apparent from the equations above, performing the energy storagefunction on the DC link capacitor rather than the decoupling capacitormay be beneficial because it typically reduces the size of the requiredcapacitor. In general, it is more economical to store energy in the formof a higher voltage on a small capacitor than a lower voltage on alarger capacitor. However, even in a conventional system that storesenergy on the DC link, the capacitor is an expensive, bulky andunreliable component that often forms the weakest link in a powerconversion system.

Moreover, sizing a capacitor for energy storage in a conventional systempresents some difficult design trade-offs. For example, even with alarge capacitor, a certain amount of ripple remains in the PV currentand/or voltage. As shown in FIG. 3, even small amounts of ripple causesignificant power losses which reduce the efficiency of the system. Theripple can be reduced by using a larger capacitor, but as shown in FIG.4, increasing the size of a capacitor dramatically increases its cost.

Power Control

Some of the inventive principles of this patent disclosure relate topower control techniques that may fundamentally alter the dynamics ofthe interface between a power converter and a power source. Some ofthese principles relate to maintaining a controlled impedance lookinginto the power converter. Referring to FIG. 5, a PV panel may be modeledas a voltage source V_(PV) and a series resistance R_(PV). The systemincludes a variable resistance R₁ that is controlled so that theimpedance Z_(IN) looking into the power converter remains constantregardless of the current I₁ that is transferred from the PV panel tothe power converter. In one example implementation, the variableresistance R₁ may be controlled by nulling the difference between theinput voltage V₁ and a reference voltage V_(REF) as shown in FIG. 6.

Some of the inventive principles involve the relationship betweenimpedance control and energy storage functions in a power converter. Forexample, in the embodiment of FIG. 7, the impedance Z_(IN) looking intoa first power converter stage 18 is maintained at a controlled value.One or more energy storage devices 20 balance the instantaneous inputpower from the power source 16 with the instantaneous output power,which may flow through one or more subsequent power stages. The powersource 16 may include a PV panel, fuel cell, battery, wind turbine, etc.The first stage 18 may include one or more DC/DC converters, DC/ACinverters, rectifiers, etc. The energy storage device may include one ormore capacitors, inductors, etc. The subsequent stages may include oneor more DC/DC converters, DC/AC inverters, rectifiers, etc.

In one example embodiment, the power source 16 includes a PV panel, thefirst stage 18 includes a DC/DC converter, and the energy storage deviceincludes a link capacitor. The impedance Z_(IN) looking into the firstpower converter stage is maintained at a constant value, while thevoltage on the link capacitor is allowed to fluctuate in response to thepulsating power demands of a subsequent stage. Because the inputimpedance control isolates the PV panel from the link capacitor, thevoltage swing on the link capacitor may be much greater than in a systemwithout impedance control. This may enable the size of the linkcapacitor to be reduced because the amount of energy stored in acapacitor is directly related to the voltage swing across the capacitor.It may also eliminate or reduce the size of a decoupling capacitor atthe input.

FIG. 8 illustrates another embodiment of a power conversion systemaccording to some of the inventive principles of this patent disclosure.The system of FIG. 8 receives power from photovoltaic cells in a PVpanel 22. The system includes a DC/DC converter 24, a link capacitorC_(DC), a DC/AC inverter 26, and a controller 28. The DC/DC convertermay include one or more stages such as buck converters, boostconverters, push-pull stages, rectifiers, etc., arranged aspre-regulators, main stages, etc. For purposes of illustration, theDC/DC converter in this example is assumed to have a pre-regulator stage24 a followed by a main stage 24 b, but the inventive principles are notlimited to such an arrangement. The DC/AC inverter 26 may include anysuitable inverter topology such as an H-bridge, a resonant inverter,etc. Voltage and current sensors 30 and 32 provide signals indicatingthe PV panel output voltage V_(PV) and current I_(PV), respectively, tothe controller 28. The controller outputs a drive signal D1 to controlthe pre-regulator.

Controller 28 implements a constant power control loop (shownconceptually by arrow 34) by controlling the pre-regulator stage 24 a inthe DC/DC converter in such a manner as to maintain the PV panel outputvoltage V_(PV) or current I_(PV) at a substantially constant value thateliminates or reduces input ripple. This causes the PV panel to see anessentially constant load which therefore results in constant powertransfer. In essence, the constant power control loop isolates the PVpanel from any stages after the preregulator 24 a, so the energy storagedevice or devices may be arranged anywhere downstream of the constantpower control loop. In the example of FIG. 8, the link capacitor is usedfor energy storage to provide cycle-by-cycle power balance at the ACoutput frequency. In other embodiments, however, the energy storage maybe located between the preregulator and the main stage, or in any otheradditional stages downstream of the constant power control loop.

Because a constant power control loop isolates the power source fromdownstream energy storage devices, the energy storage devices may beallowed to operate with wider fluctuations than would otherwise beacceptable. For example, capacitors may operate with larger voltagefluctuations, and inductors may operate with larger currentfluctuations. This, in turn, may enable the use of smaller energystorage devices.

Constant power control is distinct from, but may be used in conjunctionwith, maximum power point tracking (MPPT) according to some of theinventive principles of this patent disclosure. Whereas MPPT may seek todetermine an operating point that maximizes the power available from thepower source under certain operating conditions, constant power controlmay enable the system to maintain an operating point despitefluctuations in a load. For example, in some embodiments, MPPTtechniques may be used to find an operating point for the system,whereas constant power control techniques may be used to keep it thereas explained in more detail below with reference to FIG. 22.

Regulating a constant DC input voltage or current may provide severaladvantages. First, reducing ripple in the input waveform improves theefficiency of some DC power sources such as PV panels which suffer fromresistive losses related to the ripple. Second, moving the energystorage to the DC link capacitor may eliminate the need for an inputelectrolytic capacitor which is an expensive, bulky and unreliablecomponent with a short lifespan. Instead, the energy may be stored in ahigher voltage form on the DC link capacitor which is less expensive,more reliable, has a longer lifespan and may take up less space.Moreover, the size of the DC link capacitor itself may also be reduced.

In the example embodiment described above with respect to FIG. 8, thecontroller has one sense input (either V_(PV) or I_(PV)) and one controloutput (D1) which controls the pre-regulator in the DC/DC converter.That is, the constant power control loop is implemented by controllingthe first stage in the power path in response to a parameter sensed atthe overall input of the power converter system.

Some additional inventive principles of this patent disclosure enablethe implementation of constant power control by (1) controlling one ormore power stages other than the first stage in response to a parametersensed anywhere in the system; and/or (2) by controlling any power stageor stages in response to one or more parameters sensed anywhere in thesystem other than at the overall input.

For example, according to some of these additional inventive principles,the embodiment of FIG. 8 may be modified so that the controller 28implements a constant power control loop by controlling thepre-regulator 24 a in response to a parameter sensed at the output ofthe DC/AC inverter 26. As another example, the system of FIG. 8 may bemodified so that the controller 28 implements a constant power controlloop by controlling the DC/AC inverter 26 in response to the inputvoltage V_(PV).

FIG. 9 illustrates another embodiment of a power conversion systemhaving constant power control according to some inventive principles ofthis patent disclosure. A power path 36 includes N power stages 38,where N≧1. The power path receives power from power source 40 andoutputs power to load 42. A controller 44 receives one or more sensesignals S₁, S₂ . . . S_(L) from the power path and outputs one or moredrive signals D₁, D₂ . . . D_(M) to the power path. The power stages 38may include one or more DC/DC converters, DC/AC inverters, rectifiers,energy storage devices, etc. for processing the power as it is convertedfrom the form provided by the power source 40 to the form delivered tothe load 42. The one or more sense signals S₁, S₂ . . . S_(L) may betaken from the inputs and/or outputs of any of the power stages, frompoints within the power stages, and/or from points between the powerstages. The one or more drive signals D₁, D₂ . . . D_(M) may be arrangedto control one, any or all of the power stages or portions of the powerstages. A drive signal may be arranged to control more than one drivestage or portions of one or more drive stages in unison.

The controller 44 implements a constant power control loop using atleast one sense signal from a point other than the overall input to thepower path and/or at least one drive signal that drives at least onepower stage other than the first stage.

In some instances, providing constant power control may involvemaintaining a parameter at a constant value, for example, maintainingthe overall input voltage to the power converter system at a constantvalue. In other instances, constant power control may involvecontrolling a parameter to have a dynamic characteristic, for example,by controlling the AC voltage swing on a link capacitor to have asinusoidal waveform. In some embodiments, some stages may be leftfree-running, e.g., uncontrolled, open loop, fixed pulse width PWM,etc., while in other embodiments, some form of closed loop control maybe applied to every stage.

In some embodiments, constant power control may involve regulating thevalue of one or more sensed parameters, for example, regulating thevalue of the input voltage sensed at the input of the system. In someembodiments, the controller may use one or more additional sensedparameters as feedback signals, alone or in combination with othersensed parameters. In other embodiments, one or more additional sensedparameters may be used as feedforward signals, alone or in combinationwith other sensed parameters.

In FIG. 9, the power stages in the power path are shown generally in arow, but stages are not required to be in series. Some stages may bearranged in parallel, in series-parallel combination, or in any othersuitable configuration in accordance with the inventive principles,although at least one first stage is coupled to the overall input of thepower path.

Moreover, rather than directly regulating the input to null the ripple,ripple at an energy storage device elsewhere in the system may becontrolled to produce the same effect at the input.

FIG. 10 illustrates an embodiment of a controller for implementingconstant power control according to some inventive principles of thispatent disclosure. The controller receives one or more sense signals S₁,S₂ . . . S_(L) from one or more sense circuits which may be simple ohmicconnections, current shunts, Hall-effect sensors, bridge circuits,transformers, etc. One or more amplifier/buffer circuits 46 may be usedto condition the sense signals before they are applied to one or morecontrol blocks 48, each of which implements a function H₁(s), H₂(s) . .. H_(L)(s).

The outputs from the control blocks are applied to a control algorithmsection 50 which implements one or more control algorithms to generatethe output drive signals D₁, D₂ . . . D_(M). The one or more controlblocks 48 and/or control algorithm section 50 may be implemented inhardware, software, firmware, etc., or any combination thereof. Hardwaremay be realized with analog circuitry, digital circuitry, or anycombination thereof.

FIG. 11 illustrates an embodiment of a power converter system accordingto some of the inventive principles of this patent disclosure. DC poweris applied to the system at terminals 292 and 294. The embodiment ofFIG. 11 is shown in the context of a solar panel 290, but it may beutilized with other DC power sources such as fuel cells, batteries,capacitors, etc. In this example, the main power path continues througha collection of components that form a DC-DC converter 306. The DC-DCconverter transforms the DC power from relatively low voltage and highcurrent, which is characteristic of PV panels having crystalline cellsand some other DC power sources, to relatively higher voltage and lowercurrent suitable for conversion to AC power in a form that can be easilydistributed to a local user and/or transmitted to remote users through apower grid, etc. In other embodiments, for example, systems based onthin-film PV cells, the DC power may be generated at higher voltages,thereby eliminating or reducing the need or usefulness of voltageboosting, pre-regulation, etc. In this embodiment, the DC-DC converteris shown with two stages: a boost-type pre-regulator and a push-pulltype main stage. In other embodiments, however, the DC-DC converter maybe implemented with any suitable arrangement of single or multiplestages.

Referring again to FIG. 11, a zero-ripple input filter 296, for examplea passive filter, may be utilized to reduce high frequency (HF) ripplefor improved efficiency. Depending on the implementation, the benefit ofthe zero ripple filter may not be worth the additional cost.

Pre-regulator 298 may enable the system to operate from a wider range ofinput voltages to accommodate PV panels from different manufacturers.The pre-regulator may also facilitate the implementation of an advancedcontrol loop to reduce input ripple as discussed below. Thepre-regulator may be implemented, for example, as a high-frequency (HF)boost stage with soft switching for high efficiency and compact size. Inthis example, the pre-regulator provides a modest amount of initialvoltage boost to feed the next stage. However, other pre-regulatorstages such as buck converters, buck-boost converters, push-pullconverters, etc., may be used as a pre-regulator stage.

Push-pull stage 300 provides the majority of the voltage boost inconjunction with a transformer 302 and rectifier 304. The use of apush-pull stage may facilitate the implementation of the entire systemwith a single integrated circuit since the drivers for both powerswitches may be referenced to the same common voltage. The output fromthe rectifier stage 304 is applied to a DC link capacitor C_(DC) whichprovides a high voltage DC bus to feed the DC-AC inverter stage 312.

The inverter stage 312 includes a high voltage output bridge 308 which,in this embodiment, is implemented as a simple H-bridge to providesingle-phase AC power, but multi-phase embodiments may also beimplemented. A passive output filter 310 smoothes the waveform of the ACoutput before it is applied to a load or grid at the neutral and lineoutput terminals L and N.

A first (input) PWM controller 314 controls the pre-regulator 296 inresponse to various sense inputs. In the embodiment of FIG. 11, voltagesensors 316 and 320 and current sensor 318 provide a measure of theoverall input voltage and current and the output voltage of thepre-regulator, respectively. However, the first PWM controller mayoperate in response to fewer or more sense inputs. For example, any ofthese sense inputs may be omitted and/or other sense inputs may beincluded, e.g., the voltage on DC link capacitor C_(DC), or currentsmeasured at any other points along the power path.

As explained above, power is preferably drawn from the DC source at aconstant rate, whereas the instantaneous AC power output fluctuatesbetween zero and some maximum value at twice the AC line frequency. Toprevent these AC power fluctuations from being reflected back to the DCpower source, an energy storage capacitor is used to store energy duringtroughs (or “valleys”) in the AC line cycle, and release energy duringpeaks in the AC line cycle. This is conventionally accomplished throughthe use of a large electrolytic capacitor for the DC link capacitorC_(DC), which is held at a relatively constant value with a small amountof ripple.

In some embodiments, the first PWM controller 314 implements an innerconstant power control loop as described above (and shown conceptuallyby arrow 315) by controlling the pre-regulator 296 to maintain aconstant voltage at the input terminals 292 and 294. If the poweravailable from the PV panel is constant, then maintaining a constantpanel voltage results in constant output current from the panel as well.Alternatively, the controller may regulate the current rather than thevoltage. The constant power control loop prevents ripple on the DC linkcapacitor from being reflected back to the input. Thus, the voltageswing on the DC link capacitor may be increased and the size of thecapacitor may be reduced, thereby enabling the use of a capacitor thatis more reliable, smaller, less expensive, etc.

A maximum power point tracking (MPPT) circuit 344 forms an outer controlloop to maintain the average input voltage and current, sensed byvoltage and current sensors 316 and 318, respectively, at the optimumpoints to maximize the output power available from the DC power source,which in this example, is a PV panel.

A second (push-pull) PWM controller 324 controls the push-pull stage,which in this embodiment, operates at a fixed duty cycle. A summing node329 compares the DC link voltage from sensor 326 to a link referencevoltage LINK REF and applies the output to a link voltage controlcircuit 322. Alternatively, the output of the summing node 329 may beapplied to the third (output) PWM controller 330 to enable the outputsection to control the link voltage.

The DC-link voltage controller 322 may operate in different modes. Inone mode, it may simply allow the output from the summing node 329 to beapplied to the PWM circuit, thereby causing the DC-link voltage to beregulated to a constant value. However, if used in conjunction with theinput ripple reduction loop discussed above, the DC-link voltagecontroller 322 may filter out the AC ripple so that the third PWM looponly regulates the long-term DC value (e.g., the RMS value) of theDC-link voltage. That is, the AC ripple on the DC-link capacitor rideson a DC pedestal that slides up or down in response to the DC-linkvoltage controller. This may be useful, for example, to controldistortion in the AC output power as discussed below.

A third (output) PWM controller 330 controls the four switches in theH-bridge 308 to provide a sinusoidal AC output waveform. A non-DQ,non-cordic polar form digital phase locked loop (DPLL) 332 helpssynchronize the output PWM to the AC power line. The overall AC outputis monitored and controlled by a grid current control loop 336 whichadjusts the third PWM controller 330 in response to outputs from theMPPT circuit, the DC-link voltage controller, the DPLL, and the outputvoltage and/or current. A harmonic distortion mitigation circuit 338further adjusts the output PWM through a summing circuit 334 toeliminate or reduce distortion in response to the output voltage andcurrent waveforms sensed by voltage and current sensors 340 and 342,respectively. An output from the harmonic distortion mitigation circuitmay additionally be applied to the grid current control loop 336.

An output signal from the harmonic distortion mitigation circuit 338 mayalso be applied to the DC-link voltage controller for optimization ofthe DC-link voltage. In general, it may be preferable to minimize theDC-link voltage to increase overall efficiency. However, if the troughsof the voltage excursions on the DC-link capacitor fall too low, it maycause excessive distortion in the AC output. Thus, the DC-link voltagecontroller may slide the DC pedestal on the DC-link capacitor up or downto maintain the bottoms of the AC troughs at the lowest point possiblewhile still holding distortion to an acceptable level as indicated bythe harmonic distortion mitigation circuit.

In some alternative embodiments, the DC-link voltage controller 322 mayprovide a feedback signal which is compared to a reference signal andapplied to the second PWM controller 324, which may then control the DClink voltage by adjusting the PWM to the push-pull stage.

FIG. 12 is a schematic diagram of an embodiment of a main power pathsuitable for implementing the inverter system of FIG. 11 according tosome of the inventive principles of this patent disclosure. Power fromDC power source 346 is applied to the system at capacitor C₁ which maybe a large energy storage capacitor, or if the input ripple reductioncontrol loop is used, a smaller filter capacitor to prevent HF switchingtransients from being fed back into the DC power source. Inductor L1,transistor Q1 and diode D1 form the pre-regulation boost converter whichis controlled by the input PWM controller.

The output from the boost converter appears across capacitor C2 whichmay provide HF filtering and/or energy storage depending on theimplementation. The push-pull stage includes transistors Q2 and Q3 whichalternately drive a transformer in response to the push-pull PWMcontroller. The transformer may be a split core type T1, T2 as shown inFIG. 11, a single core type, or any other suitable configuration. Thetransformer has an appropriate turns ratio to generate a high-voltage DCbus across the DC-link capacitor C_(DC) to adequately feed the outputbridge. Depending on the implementation, the transformer may alsoprovide galvanic isolation between the input and output of the invertersystem. The rectifier may include passive diodes D2-D5 as shown in FIG.12, active synchronous rectifiers, or any other suitable arrangement.

Transistors Q4-Q7 in the HV output bridge are controlled by the outputPWM controller to generate the AC output which is filtered by gridfilter 348 before being applied to the load or power grid.

An advantage of the embodiment of FIG. 12 is that it is readilyadaptable to fabrication as an integrated power converter, for example,with a single integrated circuit (IC). Since most of the power switchesare referenced to a common power supply connection, isolated drivers arenot required for these switches. The combination of a constant powercontrol feature with a push-pull stage and a downstream energy storagedevice may be particularly beneficial because of the synergisticinteraction of components. These benefits may also extend to discreteimplementations as well.

In a monolithic implementation of the entire structure, there may bedielectric isolation between the high-side switches in the outputH-bridge and their corresponding low-side switches. There may also beisolation between different sections of the system. For example, sensecircuitry located in one section may transfer information to processingcircuitry in another section that performs control and/or communicationand/or other functions in response to the information received from thefirst section. Depending on the particular application and powerhandling requirements, all of the components including the powerelectronics, passive components, and control circuitry (intelligence)may be fabricated directly on the IC chip. In other embodiments, it maybe preferable to have the largest passive components such as inductors,transformers and capacitors located off-chip. In yet other embodiments,the system of FIG. 12 may be implemented as a multi-chip solution.

Some additional inventive principles of this patent disclosure relate tointegrating constant power control functionality into power sourcesand/or power conversion systems. In some embodiments, a constant powercontrol apparatus may be integrated into a power source at a lower levelsuch as a cell level, string level, etc. For example, in a PV panel 350as shown in FIG. 13, one or more constant power control loops 350 may beintegrated on each cell 354 on the panel. In another embodiment as shownin FIG. 14, one or more constant power control loops 356 may beintegrated on the panel 358 with each string of cells 360. In anotherembodiment as shown in FIG. 15, a single constant power control loop 362may be used for the combined output from all of the cells on the panel364. The single loop 362 may be integrated with one of the cells 366 orseparate from any cells. In another embodiment as shown in FIG. 16,multiple constant power control loops 368 may be associated with a panel370, either integrally or separately from the panel. In other examples,a constant power control loop may be integrated with each cell, eitheras one or more discrete components associated with each cell, orintegrated partially or completely on the same substrate used for eachcell. These types of integrated solutions may involve outputs frommultiple constant power control loops that may be combined in series,parallel, series-parallel combination, etc.

Some additional inventive principles of this patent disclosure relate tocontrolling power to a fluctuating value rather than a constant in apower conversion system. For example, in some embodiments, the power maybe controlled to any arbitrary function, or to a specific function thatis customized to a particular system. In other embodiments, the powermay be controlled to a dynamic value that may be synchronized withfluctuations in the power demand of a load, with fluctuations in thepower supplied by a source, a combination of both, etc.

Distortion Mitigation

Some additional inventive principles of this patent disclosure relate totechniques for mitigating distortion such as harmonic distortion in apower conversion system. Although some of the principles relating todistortion mitigation are illustrated in the context of embodiments thatalso include constant power control, the inventive principles relatingto distortion mitigation may be applied independently of constant powercontrol and other inventive principles disclosed herein.

FIG. 17 illustrates the instantaneous demand for voltage from anH-bridge type DC/AC inverter in comparison to the voltage available froma DC link capacitor that is maintained at a fixed voltage. As long asthe DC link voltage is maintained above the peak voltage demand from theinverter (plus an extra amount for headroom), the inverter can producethe AC output with little or no harmonic distortion (HD) in the outputvoltage and current waveforms.

FIG. 18 illustrates the instantaneous demand for voltage from anH-bridge type DC/AC inverter in comparison to the voltage available froma DC link capacitor that has a large AC voltage swing due to a constantpower control feature as described herein. In general, fluctuations onthe DC link voltage may cause distortion in the AC output. Moreover, atcertain points in the line cycle, minimums in the voltage available fromthe link capacitor coincide with peaks in the voltage demand from theinverter. At these points, the AC output voltage and/or current from theinverter may become excessively distorted due to a lack of adequatevoltage and headroom to the inverter. In other words, in someembodiments, the inclusion of a constant power feature may cause acertain amount of distortion in the output current, depending on theamount of AC ripple that allowed on the DC link capacitor. Harmonicdistortion is particularly troublesome for grid tie applications or anyother applications where regulations and/or specifications limit theamount of distortion in the AC output.

FIG. 19 illustrates an embodiment of a power conversion system havingharmonic distortion mitigation according to some of the inventiveprinciples of this patent disclosure. This embodiment includes a powersource 52, and a power path having a first power stage 54, an energystorage element 56 and a second power stage 58. A controller 60 imposesconstant power control on the system using one or more sense inputsobtained from any suitable point(s) in the system and one or more driveoutputs applied to any suitable point(s) in the system. A harmonicdistortion mitigation (HDM) apparatus 62 may use one or more senseinputs obtained from any suitable point(s) in the system and one or moredrive outputs applied to any suitable point(s) in the system.

The harmonic distortion mitigation block may implement one or more ofnumerous different mitigation strategies according to some of theinventive principles of this patent disclosure. One example isillustrated in the embodiment of FIG. 11. As another example, the HDMblock may take one or more sense inputs from the input and output of thesecond power stage 58 and control the first stage 54 and/or second stage58 in a manner similar to the HDM feature shown in the embodiment ofFIG. 11. The HDM functionality may be coordinated with the constantpower control functionality, or it may operate independently of theconstant power control.

FIG. 20 illustrates an embodiment of a distortion mitigation systemaccording to some of the inventive principles of this patent disclosure.The embodiment of FIG. 20 includes a power path having an energy storageelement 200, a power stage 202 and a load 204. A controller 206 receivesone or more load signals S_(L) that provide information on distortion inthe flow of power to the load. One or more control signals, for example,one or more modulation signals S_(M), enable the controller to controlthe power stage in a manner that may mitigate distortion. One or moresense signals S_(ES) from the energy storage element provide informationthat may be used to control one or more parameters of the energy storageelement. Though shown coupled to and from specific points in FIG. 20,the signals may be coupled to or from any other suitable points. Forexample, the one or more load signals S_(L) are shown originatingbetween the power stage and the load, but they may be taken directlyfrom the power stage, the load, or any other suitable location.

The controller 206 includes a control function such as modulator 210 tocontrol the power stage 202, a synchronization function 212 tosynchronize the modulator to the load, and a distortion mitigationfunction 208 to mitigate distortion in the flow of power to the load.The controller functions may be implemented in hardware, software,firmware, etc., or any combination thereof. Hardware may be realizedwith analog circuitry, digital circuitry, or any combination thereof.The implementations of the functions may be consolidated in a singleapparatus or distributed throughout multiple apparatus, etc.

The energy storage element 200 may include one or more capacitors,inductors, or any other energy storage elements. The power stage 202 mayinclude one or more DC/DC converters, DC/AC inverters, rectifiers, etc.The load may be an AC load, DC load, or any combination thereof. Thecontrol function may include any suitable type of modulation functionsuch as pulse width modulation (PWM), pulse frequency modulation (PFM),or any other suitable type of control or modulation function. Thesynchronization function 212 may include a phase-locked loop (PLL)function, delay locked loop (DLL) function, or any other suitablefunction to synchronize the control of the power stage with the load.The distortion mitigation function 208 may include harmonic distortionmitigation or cancellation, or any other type of distortion mitigation.

FIG. 21 illustrates another embodiment of a distortion mitigation systemshowing some example implementation details according to some of theinventive principles of this patent disclosure. In the embodiment ofFIG. 21, the energy storage element includes a capacitor C_(Dc) having afluctuating voltage that may be caused, for example, by a constant powercontrol. The power stage 214 includes a DC/AC inverter, which in thisexample, includes an H-bridge. The load 216 may include any type of ACload, but in this example, it is assumed to include a power distributiongrid that operates on conventional sinusoidal AC waveforms. A gridfilter 218 may be included between the H-bridge and the grid.

In this example, the controller 207 receives a link voltage V_(DC) whichis obtained from the capacitor C_(DC) by a voltage sensor or connection224. A PWM modulation signal ma2 is provided to the H-bridge from thecontroller 207. Load signals include the grid current I_(G) obtainedfrom a current sensor or connection 222, and the grid voltage V_(G)obtained from a voltage sensor or connection 220.

The controller of FIG. 21 includes a Sine PWM element 226 to a generatepulse width modulation signal Ma2 that causes the H-bridge to produce asinusoidal AC output. Although this embodiment is directed to sinusoidalwaveforms, other types of AC waveforms may be utilized in otherembodiments. The synchronization function is performed by a digitalphase-locked loop 228 that generates a phase signal θ in response to thegrid voltage V_(G). The distortion mitigation function is performed by aharmonic distortion cancelation HDC element 230 that generates amagnitude signal Ma in response to the grid current I_(G), and the phasesignal θ. The HDC element may optionally include a link voltage controlfeature that operates in response to the link voltage V_(G). The outputsfrom the HDC element and the DPLL are applied to the Sine PWM element226 which generates the modulation signal Ma2 for controlling theH-bridge. The outputs from HDC element and the DPLL may be applieddirectly to the Sine PWM element 226, or through other combinations ofelements. For example, in other embodiments, the signal Ma may not beapplied directly to the Sine PWM element 226, but instead may becombined with the output of the Sine PWM element in an adder.

The selection and arrangement of functions within the controller 207 aresubject to countless variations according to some of the inventiveprinciples of this patent disclosure. Some examples are described belowby way of example.

FIG. 22 illustrates another embodiment of a controller having harmonicdistortion mitigation according to some of the inventive principles ofthis patent disclosure. In the embodiment of FIG. 22, the HDC element230 includes a sine generator 234 which generates a sine signal sin(θ)in response to the phase signal θ from the DPLL. The signal sin(θ) iscombined with a reference signal I_(REF) by multiplier 236 to generate ascaled signal I_(REF) sin(θ) that is compared to the grid current I_(G)by adder (or comparator) 238 to generate an error signal I_(ERR). Theerror signal may be subjected to a transfer function H(s) by functionblock 240 to generate the error magnitude signal MAG.

The embodiment of FIG. 22 implements a direct method of controlling thepower stage where the grid current is compared to the scaled sinusoidalsignal I_(REF) sin(θ). The resulting output Ma2 from the Sine PWM 226has the form Ma2=MAG·sin(θ). In operation, the MAG portion may exhibitdistortion as a function of time as the control loop tries to maintain apurely sinusoidal output despite the presence of ripple on the linkvoltage. The system's ability to mitigate harmonic distortion may dependon the bandwidth of the path including the comparator 238, function 240,and Sine PWM 226, which form a loop with the H-bridge and grid filter ifpresent. This loop will typically cancel out harmonics at frequenciesbelow the bandwidth of the loop, for example, an order of magnitudelower. Thus, the path including the comparator, H(s), and Sine PWM mayform a relatively fast inner loop, whereas the path including the DPLL228 and sine generator 234 may form a slower outer loop.

The reference signal I_(REF) may be a fixed reference signal.Alternatively, as shown in FIG. 22, I_(REF) may be provided by a DC linkvoltage control feature 242 as part of another control loop to controlthe DC link voltage. The link voltage V_(DC), or an average or RMSversion of V_(DC), may be compared to a reference signal V_(REF) togenerate I_(REF). The DC link voltage control may be implemented asanother relatively slow outer control loop.

Some additional inventive principles of this patent disclosure relate togrid current control. The embodiment of FIG. 23 includes grid currentcontrol element 244 to generate direct and quadrature signals I_(D) andI_(Q) in response to the grid current I_(G) and the phase signal.theta.from the DPLL, as well as a reference signal I_(REF2). The direct andquadrature signals are applied to an inverse DQ transform element 246which generates a phase signal φ, which is applied to sine generator234, and a magnitude signal MAG′, which is applied to Sine PWM 226. Theoutputs MAG and MAG″ from the HDC block 230 and Sine PWM 226,respectively, are combined by an adder 248 to provide the finalmodulation signal Ma2.

By providing grid current control, the embodiment of FIG. 23 may beconfigured to force the grid voltage V_(G) and grid current I_(G) into acloser phase relationship. For example, the previous embodiment of FIG.22 may provide adequate operation in systems having a purely or mostlyresistive grid load. In a system having a grid load with reactivecomponents, the grid current control feature of the embodiment of FIG.23 may force the grid voltage and current to be in phase, therebyproviding improved harmonic distortion cancellation with a reactivegrid.

Although shown in conjunction with the HDC feature 230 in FIG. 23, thegrid current control techniques disclosed herein may be implementedseparately from this or any other HDC features according to some of theinventive principles of this patent disclosure.

The grid current control techniques illustrated in the context of FIG.23 may also be combined with various forms of link voltage control. Forexample, either or both of the reference signals I_(REF1) and I_(REF2)may be provided by one or more link voltage control elements such aselement 242 shown in FIG. 22.

Some additional principles of this patent disclosure relate to the useof predistortion techniques for distortion mitigation. FIG. 24illustrates an embodiment of a controller 250 having a control functionsuch as a modulator 210 to generate one or more control signals S_(M) tocontrol a power stage, and a synchronization function 212 to synchronizethe output of the power stage with a load in response to one or moreload signals S_(L). A predistortion element 252 provides some form ofpredistortion in response to any suitable signal such as a sense signalS_(ES) from an energy storage element. The predistortion may be appliedto the one or more control signals S_(M) or any other signal or elementto provide distortion mitigation. The controller functions may beimplemented in hardware, software, firmware, etc., or any combinationthereof. Hardware may be realized with analog circuitry, digitalcircuitry, or any combination thereof. The implementations of thefunctions may be consolidated in a single apparatus or distributedthroughout multiple apparatus, etc.

FIG. 25 illustrates an embodiment of a controller having predistortionaccording to some of the inventive principles of this patent disclosure.A modulation signal Ma may be provided by any suitable source, forexample any of the Ma2 signals in the embodiments disclosed above. Inthis example, the modulation signal Ma is provided by a simple Sine PWMelement 258 which is controlled by a DPLL 260 in response to the gridvoltage V_(G). A predistortion element 254 generates a predistortionsignal Ma′, which is combined with the modulation signal Ma by adder 256to generate the final modulation signal Ma″. The final modulation signalMa″ may be applied to any suitable power stage. In this example, thepower stage may be an H-bridge as illustrated in FIG. 21.

Predistortion methods according to some of the inventive principles ofthis patent disclosure may be implemented separately from, or inaddition to, the other types of distortion mitigation principlesdisclosed herein. The predistortion element 254 may implement any typeof predistortion to mitigate or cancel distortion in the power flow froma power stage to a load. For example, if applied to the system of FIG.21, the predistortion element 254 may generate predistortion signal Ma′that anticipates, and compensates for, the distortion caused by rippleon the link voltage V_(DC).

FIG. 26 illustrates an embodiment of a predistortion element accordingto some of the inventive principles of this patent disclosure. Theembodiment of FIG. 26 includes a look-up table 262 to provide thepredistortion signal Ma′ in response to the instantaneous link voltageV_(DC) and an average value of the link voltage V_(DC(AVERAGE)).

FIG. 27 illustrates another embodiment of a predistortion elementaccording to some of the inventive principles of this patent disclosure.The embodiment of FIG. 27 generates the predistortion signal Ma′ bydividing the average value of the link voltage V_(DC(AVERAGE)) by theinstantaneous value V_(DC). The result may be used directly as thepredistortion signal or subjected to additional processing. For example,the result may be multiplied by the modulation signal Ma aftertransformation by a function f(s) as shown in FIG. 27.

FIG. 28 illustrates another embodiment of a predistortion elementaccording to some of the inventive principles of this patent disclosure.The embodiment of FIG. 28 includes a look-up table 264 to provide thepredistortion signal Ma′ in response to the instantaneous link voltageV_(DC), an average value of the link voltage V_(DC(AVERAGE)), theinstantaneous grid voltage V_(G), and the RMS value of the grid voltageV_(G(RMS)).

FIG. 29 illustrates another embodiment of a predistortion elementaccording to some of the inventive principles of this patent disclosure.The embodiment of FIG. 29 calculates the predistortion signal Ma′ inresponse to the instantaneous link voltage V_(DC), an average value ofthe link voltage V_(DC(AVERAGE)), the instantaneous grid voltage V_(G),and the RMS value of the grid voltage V_(G(RMS)) according to anysuitable transfer function H(s).

In some applications, the embodiments illustrated with respect to FIGS.26 and 27 may provide adequate distortion mitigation where the grid loadhas purely or nearly sinusoidal waveforms. In other applications, theembodiments illustrated with respect to FIGS. 28 and 29 may providebetter distortion mitigation where the grid load waveforms containsignificant amounts of distortion.

The inventive principles relating to predistortion are not limited tosystems having sinusoidal AC loads. A predistortion signal Ma′ may begenerated to compensate for distortion in loads having waveforms such astriangle waves, sawtooth waves, square waves, etc. In embodiments havinglook-up tables, the look-up tables may be static, or they may changeover time, for example, in response to various inputs such as linevoltage, frequency, link voltage, or any other operating parameter.Distortion mitigation techniques according to the inventive principlesmay also be implemented using any suitable algorithms including somefrom the audio industry which may be applied directly or adapted for usewith the inventive principles.

The various inventive principles relating to distortion mitigation mayall be utilized separately, or in combination with other inventiveprinciples. For example, in some embodiments, a controller may combinepredistortion with link voltage control, while in other embodiments, acontroller may combine direct harmonic distortion cancellation with gridcurrent control, predistortion and link voltage control according tosome of the inventive principles of this patent disclosure.

Impedance Transformation

Some additional inventive principles of this patent disclosure relate totechniques for manipulating a constant power control loop to provideimpedance transformation, to determine a maximum power point or otheroperating point, and/or for other purposes.

Referring to FIG. 30, a PV panel is modeled as a voltage sourceV_(INTERNAL) and a series resistance R_(INTERNAL). A constant powercontrol loop causes the PV panel to see a constant load I_(PV) with aconstant input impedance of Z_(IN)=V_(PV)/I_(PV). Due to the impedancetransformation, the constant power applied to load I_(PV) is transformedto a constant power delivered to the DC link. The power P is constantand equal to V_(PV)*I_(PV). Since the power is constant, and the currentdrawn by the H-bridge varies at twice the line frequency, the linkvoltage V_(DCLINK) must also vary at twice the line frequency becausethe product of the current and voltage must be constant. Thus, thecurrent delivered to the DC link varies asP/V_(DCLINK)=V_(PV)*I_(PV)/V_(DCLINK).

Power transfer from the PV panel to the converter system is maximizedwhen the series resistance of the panel R_(INTERNAL) matches theimpedance presented by the load I_(PV), that is, whenZ_(IN)=R_(INTERNAL)=V_(PV)/I_(PV).

In some embodiments, a system that implements constant power control asdescribed above may transform a DC/DC converter or other power stagefrom a low AC impedance path to a high AC impedance path. This can bebetter understood with reference to FIG. 31 where an AC load isillustrated as a current source 100 which draws a pulsating currentI_(AC). A conventional DC/DC converter is illustrated as a low impedancepath 102. If either of capacitors C₁ or C_(DC) has a large value, itforms a low impedance path to the common node, and therefore, thepulsating current I_(AC) is blocked from being reflected back to the PVpanel. If, however, capacitors C₁ and C_(DC) are removed or reduced insize, the DC/DC converter forms a low impedance AC path between the load100 and the PV panel. Thus, the pulsations in the current I_(AC) show upas voltage and/or current fluctuations at the output of the PV panel.

Implementing a constant power control loop in the DC/DC converter,however, may cause the converter to appear as a path having a high ACimpedance. Therefore, the pulsating AC current I_(AC) is prevented fromflowing through the DC/DC converter. As a result, all or most of the ACcurrent must flow through the link capacitor C_(DC), which, because ofits low capacitance, causes a large voltage swing across C_(DC).

Because the impedance transformation properties of constant powercontrol may be the result of control operations rather than hardwiredcomponents, they may be changed rapidly and/or in a controlled manner.For example, the flow-through impedance of the DC/DC converter can bechanged instantaneously by the controller. Such properties may beexploited to provide some beneficial results.

Operating Point Sweep

One such application involves determining a maximum power point or otheroperating point for a power source coupled to a power converteraccording to some of the inventive principles of this patent disclosure.

Referring to FIG. 32, curve 104 illustrates the voltage-currentcharacteristic (V-I curve) of a typical PV panel under certain operatingconditions, while curve 106 illustrates the corresponding powercharacteristic (power curve) for the same panel under the sameconditions. The V-I curve is zero volts with a value of I_(SC) which isthe short-circuit current generated by the panel when the outputterminals are shorted together. As the output voltage increases, the V-Icurve remains at a fairly constant level of current until it reaches aknee at which it descends rapidly toward zero current at V_(OC), whichis the open-circuit output voltage of the panel.

The power curve is simply the current times the voltage at each pointalong the V-I curve. The power curve has a maximum value correspondingto a certain voltage level and a certain current level. This is known asthe maximum power point or MPP. Most PV power systems attempt to operateat the maximum power point. The maximum power point, however, tends tochange based on changes in operating conditions such as illuminationlevel, temperature, age of the panel, etc. Therefore, algorithms havebeen devised for tracking the MPP as it changes over time.

Existing algorithms for maximum power point tracking (MPPT) aregenerally slow processes that are performed over a relatively long timeframe compared to the period of an AC line cycle. Moreover, existingalgorithms assume that only one MPP exists in the power curve. Powercurves for some PV panels and other power sources, however, may havemultiple local maximum points. One example is shown in FIG. 33 whichillustrates V-I and power curves for a power source having more than onelocal MPP. A conventional MPPT algorithm might approach local maximumMPP1 from the left and stop once it determines that the power curve istrending back down as it moves to the right. In this case, the algorithmwould erroneously determine that MMP1 is the maximum power point ratherthan MMP2, which is the true maximum power point. An existing algorithmmay be modified by forcing it to continue searching through the rest ofthe voltage range, but with existing techniques, this would be a lengthyprocess.

In a power conversion system having constant power control, the controltechnique may be manipulated to provide maximum power point tracking orother techniques according to some additional inventive principles ofthis patent disclosure. As explained above, a constant power controlloop may prevent power pulses from being reflected back to a powersource. This is illustrated in FIG. 34 where a power stage 108 iscontrolled by a constant power control loop 110 which prevents powerpulses from AC load 112 from reaching the power source 114.

By selectively disabling or otherwise modifying the constant powercontrol loop, some or all of the power pulsations may be reflected backto the power source in a manner that can be observed for purposes ofdetermining an operating point or other useful information. For example,in FIG. 35, the control loop 110 is disabled by SW1. This causes thepower stage to operate in some other mode, for example, at a fixed dutycycle, thereby allowing power pulsations from the AC load to reach thepower source. A tracking circuit 116 measures the resulting voltageand/or current fluctuations in the output from the power source 114 anduses this information to implement an MMPT algorithm or other processes.The use of a relatively small energy storage devices such as a smallcapacitor may enable the power pulsations from the AC load to reach thepower source. If, for example, a larger capacitor is used, it may blockthe pulses from reaching the source.

FIG. 36 illustrates how the embodiment of FIGS. 34 and 35 may operateunder some conditions. The system is initially assumed to be operatingat point B with the constant power control loop enabled. The controlloop is then disabled to allow the power stage 108 to operate open loopat a fixed duty cycle. The power pulsations from the AC load arereflected back to the power source, thereby causing the operating pointto ride back and forth along the power curve between points A and C asthe output voltage and current from the power source sweep through thecorresponding ranges V_(SWEEP) and I_(SWEEP). The tracking circuit 116monitors the output voltage and current and can therefore calculate thepower at every point between A and C. Since the swept range includesboth local maximums MPP1 and MPP2, the tracking circuit can compare themto determine the true MPP.

In this example, the true MPP is found to be at point B′. Once the MPPis determined, the constant power loop may be re-enabled to cause thesystem to remain at B′ regardless of fluctuations in the AC load. In theabsence of the constant power control loop, fluctuations in the AC loadwould cause the operating point to fluctuate around point B′ as shown bythe arrows in FIG. 36.

The tracking operation described above may provide a fast and robusttechnique for determining the MPP or other operating point because itmay sweep a large operating range on a smaller time scale than istypically employed in MPPT routines, in some cases in less than a linecycle of the AC load. For example, in a system with a sinusoidal output,the information in the first phase is the same as the second phase.Therefore, in a system with a 60 Hz sinusoidal output only a half cycleof the 120 Hz power ripple is needed to obtain all of the information.Thus, the sweep can be conducted in ¼ of a 60 Hz line cycle or ˜4 ms.

Implementation may be fast and simple because the constant power controlloop may be easily enabled, disabled or otherwise modified in a controlalgorithm. During the sweeping process, perturbations are provided bythe AC load, thereby reducing or eliminating the need for additionalcircuitry to create perturbations.

The process may also be highly flexible and adaptable to countlessvariations in numerous parameters. For example, the power stage may beset to any suitable fixed duty cycle or other operating mode during thesweeping operation. Alternatively, the duty cycle may be stepped throughdifferent values to extend the sweep range over the course of multiplecycles of the AC load. The system may be configured to sweep the entireoperating range of the power source, or fixed or flexible bounds may beplaced on the sweep range. For example, in some embodiments, a sweepoperation may simply be allowed to sweep whatever range is provided bythe particular AC load using a particular fixed duty cycle in the powerstage. In other embodiments, limits may be set in the output voltageand/or current from the power source. For example, if a high or lowlimit is reached, the constant power control loop may be enabled, eitherat the original operating point (B), or at some revised operating point,e.g., the limit itself. Thus, the control loop itself may be used tolimit the swing through the V-I and power curves if the entire dynamicrange does not need to be sampled.

A sweep operation may be initiated in response to various eventsaccording to some of the inventive principles. In some embodiments,sweep operations may be initiated at periodic time intervals, e.g., onceevery second or few seconds, once every minute or few minutes, etc. Inother embodiments, a sweep operation may be triggered when a monitoringoperation determines that the system is not operating where it wouldnormally be expected to operate. A sweep operation may alternatively beinitiated by an external stimulus.

In the example embodiment of FIGS. 34 and 35, the AC load itself is usedto create fluctuations at the power source, but other apparatus may alsobe employed to create calibrated fluctuations. For example, acontrollable load may be substituted for the normal AC load to providefluctuations at a controlled rate, and within controlled bounds.Alternatively, the controllable load may provide one or more discreteload points rather than sweeping through every point within a range. Acontrollable load may be controlled independently, or by the samecontrol circuitry used for the tracking operations.

Any or all of these features may be implemented in a dedicatedcontroller or logic, or in a controller or logic that may implementother features of the power conversion system.

Multiple Power Sources with Individual Power Control

Some additional inventive principles of this patent disclosure relate tothe use of power control in systems having multiple power sources. FIG.37 illustrates an embodiment of a system in which N multiple powersources 118 are each coupled to a corresponding one of N powerconverters 120. The outputs of the power converters are combined bycombiner 122 and applied to at least one energy storage device 124. Theoutputs of the power converters may be combined in series, parallel,series-parallel combination, or any other suitable arrangement. Thepower sources include photovoltaic devices, fuel cells, batteries, windturbines, or any other power sources or combinations thereof. The powerconverters may include one or more stages of DC/DC converters, DC/ACinverters, rectifiers, etc., or any combination thereof. One or more ofthe power converters include constant power control functionality 126.

FIG. 38 illustrates an embodiment of a power conversion system in whichmultiple DC/DC converters include constant power control functionalityaccording to some of the inventive principles of this patent disclosure.In the embodiment of FIG. 38, the power sources are implemented as PVpanels 128, each of which provides power to a corresponding DC/DCconverter 130. The outputs of the DC/DC converters are arranged inseries to generate a DC link voltage V_(d) which is applied to a linkcapacitor C_(DC). A DC/AC inverter 132 converts the link voltage to anAC voltage V_(GRID).

Each of the DC/DC converters 130 implements a constant power controlloop 134 to maintain constant power transfer from its associated PVpanel. Each of the DC/DC converters 130 may also implement a maximumpower point tracking function (MPPT) which operates as a slower outercontrol loop around the relatively faster inner constant power controlloop. Each DC/DC converter outputs a constant power that corresponds tothe input power provided by each of the individual power sources. Thelink capacitor C_(DC) operates as a combined energy storage element forall of the DC/DC converters. The link voltage Vd includes an AC ripplecomponent on top of a DC component, where the amount of AC rippledepends on the size of the link capacitor as discussed below. The outputvoltage and current from each DC/DC converter is allowed to float sothey can settle in to values that balance the voltage and currentconstraints of the entire power system. Because the converters 130 arearranged in series in this example, the output current through eachDC/DC converter must be equal, while the sum of the output voltages mustequal the DC link voltage V_(d). Other embodiments may be arranged fordifferent constraints. For example, in an embodiment where the DC/DCconverters are connected in parallel, each converter may provide adifferent amount of current.

The system of FIG. 38 may also include a link voltage control functionthat modulates the demand from the DC/AC inverter to maintain theaverage or RMS value of the link voltage at a level that providesoptimum operation of the DC/AC inverter and/or prevents or reducesharmonic distortion at the output.

Because each of the DC/DC converters 130 implements an individualconstant power control loop, the input ripple at each converter may beoptimally minimized for each PV panel. By adding MPPT functionality toeach converter, the power output from each PV panel may also beoptimized regardless of differences in the operating conditions for eachpanel, e.g., illumination conditions, temperature, age, etc.

Moreover, the size of the link capacitor C_(DC) may be reduced dependingon the implementation details. For example, in an embodiment having aDC/AC inverter 132 with a harmonic distortion mitigation feature, it maybe possible to reduce the size of the link capacitor. Even though theuse of a smaller capacitor results in larger voltage fluctuations on thelink capacitor, the presence of a harmonic distortion mitigation featuremay reduce distortion in the AC output to an acceptable level. In anembodiment with a conventional DC/AC inverter without harmonicdistortion mitigation, however, it may still be necessary to use arelatively large link capacitor because a large ripple voltage on thelink capacitor may cause unacceptable levels of distortion in the ACoutput.

Some additional inventive principles of this patent disclosure relate topower conversion system architectures that may be realized using some orall of the other inventive principles disclosed herein, alone or invarious combinations. Some of these architectures will be described withreference to the following drawings.

FIG. 39 illustrates an embodiment in which multiple modules 400, some orall of which include constant power control 402, are arranged in seriesto generate a DC link V_(LINK) that is applied to a conventional centralinverter 404. Because a conventional central inverter 404 is used, arelatively large link capacitor C_(LINK) is utilized to limit the ACripple and provide a constrained DC link that prevents excessivedistortion in the AC output.

FIG. 40 illustrates an embodiment in which multiple modules 400, some orall of which include constant power control 402, are arranged inparallel.

FIG. 41 illustrates a parallel-series embodiment in which multiplemodules 400 are arranged first in parallel units. The parallel units arethen arranged in series to provide the DC link V_(LINK).

FIG. 42 illustrates a series-parallel embodiment in which multiplemodules 400 are arranged first in serial units or strings. Theindividual strings are then arranged in a parallel combination togenerate the DC link V_(LINK).

In each of the embodiments of FIGS. 39-42, the modules may beimplemented with various alternative structures, for example, in someembodiments, each module 400 may be one or more solar panels, fuel cellsor other power sources that have the constant power control 402integrated in the source. In other embodiments, the modules may includeone or more power sources plus an associated DC/DC converter where theconstant power control 402 may be part of the DC/DC converter. Othermodule configuration are possible, including a combination of thosedisclosed herein.

Also in each of the embodiments of FIGS. 39-42, because a conventionalcentral inverter 404 is used, a relatively large link capacitor C_(LINK)is utilized to limit the AC ripple and provide a constrained DC linkthat prevents excessive distortion in the AC output.

Some additional inventive principles relate to the use of harmonicdistortion mitigation in a central inverter or other power stage incombination with one or more power sources having constant powercontrol.

FIG. 43 illustrates an embodiment in which a central inverter based onan inverter bridge 406 includes harmonic distortion mitigation 408according to the inventive principles of this disclosure. In thisembodiment, the DC link V_(LINK) may be generated by one or more powersources, some or all of which include constant power control. Forexample, any of the power source arrangements illustrated in FIGS. 39-42may be used to generate V_(LINK). However, because the H-bridge 406includes harmonic distortion mitigation 408, the ripple constraints onV_(LINK) may be relaxed, and therefore, a smaller link capacitor may beused. That is, larger voltage fluctuations may be allowed on V_(LINK)without causing excessive distortion in the AC output due to theoperation of harmonic distortion mitigation 408. Thus, the one or morepower sources having constant power control may be allowed to generate arelaxed DC link on a smaller capacitor.

FIG. 44 illustrates another embodiment of a central inverter that mayoperate with a relaxed DC link. In this embodiment, the inverterincludes a push-pull stage 410 followed by a transformer 412 to provideisolation, a rectifier 414, and an inverter bridge 406. The inverterbridge includes harmonic distortion mitigation 408. In this embodiment,energy storage may be provided by a relatively small DC link capacitorC_(LINK) that is arranged between the rectifier and the inverter bridge.In another embodiment, the link capacitor may be arranged in front ofthe push-pull stage as shown in FIG. 45. In yet other embodiments, theenergy storage may be distributed between multiple locations. Any ofthese embodiments may also include other power stages such aspreregulators, etc. Likewise, MPPT may be included at any suitablepoint, for example, at the input to the push-pull stage or the inverterbridge.

FIG. 46 illustrates another embodiment of a central inverter that mayoperate with a relaxed DC link. In this embodiment, the relaxed DC linkinput is applied to a DC/DC converter 416, which may also includeconstant power control 418. The output of the DC/DC converter stage isapplied to an inverter bridge 406 having distortion mitigation 408. Arelatively small DC link capacitor C_(LINK) may be arranged between theDC/DC converter and the inverter bridge.

FIG. 47 illustrates another embodiment of a central inverter that mayoperate with a relaxed DC link. In this embodiment, the relaxed DC linkinput V_(LINK) is applied to a line-frequency inverter bridge 407 havingdistortion mitigation 409. The output from the inverter bridge isapplied to a line-frequency transformer 411 that couples the inverter toa power grid or other AC load.

Some additional inventive principles relate to the use of constant powercontrol and harmonic distortion mitigation in a central inverter orother power stage or stages in combination with one or more conventionalpower sources.

FIG. 48 illustrates an embodiment of a central inverter that may operatewith an input directly from one or more power sources such as PV panels,fuel cells, etc. The one or more power sources generate a DC bus V_(BUS)that is input to a DC/DC converter 420 having constant power control422. The DC/DC converter is followed by a push-pull stage 410, atransformer 412, a rectifier 414 and an inverter bridge 406 havingdistortion mitigation 408.

A relatively small capacitor may be used for the link capacitor C_(LINK)because the constant power loop prevents ripple on the capacitor frombeing reflected back to the one or more power sources, while thedistortion mitigation may prevent or reduce distortion on the AC outputcaused by ripple on the DC link. Thus, a relaxed DC link may be used.

In the embodiment of FIG. 48, the link capacitor C_(LINK) is arrangedbetween the rectifier and the inverter bridge, but in other embodiments,the link capacitor may be arranged between the DC/DC converter 420 andthe push-pull stage 410 as shown in FIG. 49, or any other suitablelocation in the inverter.

In the embodiments of FIGS. 48 and 49, an MPPT function may also beimplemented in the DC/DC converter, at the inverter bridge, or elsewherein the inverter to set an operating point for the constant powercontrol, thereby maximizing the power transfer from the power source orsources on the DC bus.

The inventive principles described with respect to the embodiments ofFIGS. 44-49 may be applied to systems with central inverters,distributed inverters, combinations thereof, etc.

Some additional inventive principles relate to the use of distortionmitigation with distributed inverters, also referred to asmicroinverters or nanoinverters.

FIG. 50 illustrates an embodiment of a system in which multipledistributed inverters 424 receive power directly from multiple powersources 430. Some or all of the inverters include constant power control426 and distortion mitigation 428. The outputs from the distributedinverters are combined to provide an AC output to a grid or other ACload.

FIG. 51 illustrates an embodiment of a system in which multipledistributed inverters 432 receive power from multiple power sources 436.Some or all of the power sources include constant power control 438 toprovide a relaxed DC link to the inverters, and some or all of theinverters include distortion mitigation 434 to prevent ripple on arelaxed DC link from causing unacceptable distortion in the AC output.The outputs from the distributed inverters are combined to provide an ACoutput to a grid or other AC load.

In the embodiments of FIGS. 50 and 51, an MPPT function may also beimplemented in the distributed inverters or at any of the power sourcesto set an operating point for the constant power control, therebymaximizing the power transfer from the power source or sources.

Applications

Although some of the inventive principles of this patent disclosure havebeen described in the context of some specific embodiments relating toDC-to-AC inverter systems, the inventive principles have broadapplicability to a wide range of power conversion systems where systemssee dynamic loads and/or dynamic power sources, and therefore, requireenergy storage devices to balance the flow of power from source to load.The inventive principles may be especially advantageous wherereliability is important and energy storage devices have conventionallybeen unreliable. Some examples of suitable applications include:electric and hybrid cars, fork lifts, people movers, tramways,metro-systems; air cooling systems; solar- and wind-energy systemsincluding inverter/converter boxes; energy storage (battery) decoupling;power supplies of all kinds; motors drives of all kinds; energyconversion systems such as battery chargers and charge controllers;induction heating; EMI reduction filters including high-voltageapplications; etc.

Moreover, some of the inventive principles relating to constant powercontrol have been described in the context of embodiments havingrelatively steady power sources and fluctuating loads, but constantpower control may also be applied to systems having fluctuating powersources and relatively steady loads. They may also be applied to systemshaving both fluctuating power sources and fluctuating loads with arelatively steady power link between the source and load. In general,constant power control may be applied to isolate one or more portions ofa having relatively steady power from one or more portions havingfluctuating power.

For example, the inventive principles relating to constant power controlmay be applied to energy conversion: (a) from DC to AC such as fromsolar to grid, fuel cell to grid; (b) from AC to DC such as grid tobattery; (c) from AC to a variable mechanical load such as AC to allkinds of motors, e.g., on a production line; (d) from DC to a variablemechanical load such as from battery to electric motor in electricvehicles (EVs); (e) from a variable mechanical generator to an AC loadsuch as from a wind turbine to a grid; (f) from a variable mechanicalgenerator to a DC load such as from a wind turbine to a battery; etc. Insome embodiments, a mechanical load can be a heat load, e.g., ininduction heating.

Another illustrative example involves the application of the inventiveprinciples relating to constant power control to a wind turbine thatfeeds a grid or other AC load. If the wind flow is constant, i.e., atype of laminar flow versus turbulent flow, then the power harvested isuniform. This may be analogized to constant irradiation on a PV panel.Thus, the uniform power flow has to be stored on cycle-by-cycle basisfor transfer to the AC grid. This may be analogized to thecycle-by-cycle power storage to transfer DC power from a PV panel to anAC load.

On the other hand, if the wind flow is turbulent, then the powerharvested is dynamic, which may be analogized to a shadowing effect onthe PV panel, but possibly much faster and with more variation. In thissituation, the inventive principles relating to combining fast MPPT withcycle-to-cycle energy storage may be utilized with beneficial effects.That is, fast MPPT may be used to determine the best operating point atfrequent intervals, while a constant power control loop may be utilizedto maintain the system at the most recently determined operating point.

FIG. 53 illustrates another embodiment of a power conversion systemaccording to the inventive principles of this patent disclosure. A powerpath 148 transfers power from a power source 154 to a load 156. Thepower path includes an energy storage device 150 and a power stage 152.A controller 158 causes the power stage to control power to or from theenergy storage device. The power may be controlled to a constant value,a fluctuating value, etc. The power from the power source may have aconstant value, fluctuating value, etc. The load power may have aconstant value, fluctuating value, etc.

Some additional inventive principles of this patent disclosure relate tomitigation of electromagnetic interference (EMI).

FIG. 52 illustrates an embodiment of a power conversion system havingEMI mitigation according to the inventive principles of this patentdisclosure. A power path 140 having one or more power stages transferspower from a power source 138 to a load 142. A constant power control144 causes the power path to present a constant input impedance to thepower source. An EMI mitigation element 146 operates on the power pathto reduce or eliminate EMI that originates in the power path.

The inventive principles of this patent disclosure have been describedabove with reference to some specific example embodiments, but theseembodiments can be modified in arrangement and detail without departingfrom the inventive concepts. For example, some embodiments have beendescribed in the context of delivering power to an AC grid, but theinventive principles apply to other types of loads as well. As anotherexample, some embodiments have been described with capacitors as energystorage devices, and fluctuating DC link voltages, but the inventiveprinciples apply to other types of energy storage devices, e.g.,inductors which my provide a DC link current having an AC ripple currentinstead of voltage. As another example, any of the constant powercontrol techniques described herein may also be implemented withfluctuating power control, or any other type of power control. Suchchanges and modifications are considered to fall within the scope of thefollowing claims.

1. A system comprising: a converter to transfer power between a powersource and a load having a fluctuating power demand; a controller toprovide power control; and a distortion mitigation circuit.
 2. Thesystem of claim 1 where: the converter comprises an energy storagedevice; and the distortion mitigation circuit controls a parameter ofthe energy storage device.
 3. The system of claim 2 where the distortionmitigation circuit may slide a DC portion of the parameter to preventextremes of an AC portion of the parameter from causing unacceptabledistortion.
 4. The system of claim 1 where the distortion mitigationcircuit comprises a sine generator.
 5. The system of claim 1 where thedistortion mitigation circuit comprises a predistortion circuit.
 6. Thesystem of claim 1 where the controller comprises grid current control.7. A system comprising: a converter to transfer power between a powersource and a load having fluctuating power demand; and a controller toprovide power control; where the controller may selectively disable thepower control.
 8. The system of claim 7 where: the power controlprevents power fluctuations from reaching the power source; anddisabling the power control enables power fluctuations to reach thepower source.
 9. The system of claim 7 further comprising a trackingcircuit to monitor the power source.
 10. The system of claim 9 where thetracking circuit determines an operating point when the power control isdisabled.
 12. The system of claim 10 where the operating point comprisesa maximum power point.
 13. The system of claim 9 where the power controlis disabled periodically, or when an operating parameter of theconverter deviates from an expected value.
 14. The system of claim 13where the controller may re-enable the power control when an operatingparameter of the converter exceeds a limit.
 15. A system comprising: apower path having a first power stage coupled to an input of the powerpath; and a controller to generate a drive signal to provide powercontrol in response to a sense signal from the power path; where thesense signal is taken from other than the input of the power path, orthe drive signal is applied to the power path at other than the firstpower stage.
 16. The system of claim 15 where the sense signal is takenfrom an output of the power path.
 17. The system of claim 15 where poweris controlled by controlling a parameter of the power path to a constantvalue.
 18. The system of claim 15 where power is controlled bycontrolling a parameter of the power path to a fluctuating value. 19.The system of claim 15 where the power path includes: an energy storagedevice following the first power stage; and a second power stage havingan input coupled to the energy storage device.
 20. The system of claim19 where the drive signal is applied to the second power stage.